Cast a function pointer to a
GSourceFunc, suppressing warnings from GCC 8
-Wcast-function-type enabled about the function
types being incompatible.
For example, the correct type of callback for a source created by
GChildWatchFunc, which accepts more arguments
GSourceFunc. Casting the function with
(GSourceFunc) to call
g_source_set_callback() will trigger a warning, even though it will be cast
back to the correct type before it is called by the source.